Spent photographic processing solutions such as developers, fixers, bleach-fix, bleach, stabilizers, super stabilizers, washing aids, and wash water contain dissolved silver ions which can be economically recovered as metallic silver. Various types of apparatus have been disclosed, many of which rely on reduction of silver ions by an active metal having a higher electromotive potential than silver, such as iron. The sacrificial metal is oxidized and dissolved when the silver ions are reduced. Typically, these devices include a flow-through reaction zone in a chamber packed with active metal in finely divided form such as powder, turnings, filings, chips, wound wire, woven wire, wool, or chopped wool. Solution enters the apparatus through an inlet, deposits silver and dissolves iron while passing through the metal media in the reaction zone, and discharges through an outlet to drain. Eventually, the media is consumed or fouled so that exchange no longer occurs. The media is then replaced with fresh material, and the silver-rich precipitate is sent to a smelter.
As metal media is consumed, a reaction sludge forms which tends to plug progressively the microscopic pathways through the media. This reduces the surface area available for further reaction, reduces the effective volume of the media, increases the flow velocity, and shortens the residence time of the solution in the reaction zone. In response to these higher solution velocities, preferred flow channels can develop in the media and exchange efficiency can decrease rapidly, resulting in premature failure of the apparatus.
Many of the known apparatus designs purport to improve reaction rate, efficiency, and useful life by minimizing this well-known and serious problem of media channelling, but do not recognize fully a root cause of the problem, which is nonuniform distribution of solution pressure and flow at the entry to the media.
Many known designs use a perforated plate or grid near the inlet to support the media and to provide solution access to the media. See, for example, U.S. Pat. No. 5,004,212 to Gutierrez, Apr. 2, 1991; U.S. Pat. No. 3,792,845 to Larson et al., Feb. 19, 1974; U.S. Pat. No. 3,705,716 to Hendrickson, Dec. 12, 1972; U.S. Pat. No. 3,369,801 to Hartman, Feb. 20, 1968; and U.S. Pat. No. 2,194,056 to Quaglia, Mar. 19, 1940. Such a plate has a relatively small and finite number of holes and is by its nature a discontinuous distributor, having the web of the plate between the holes. Channelling and consequent plugging develop in the media in the vicinity of each hole in the plate, and media between the holes becomes shielded from further exposure to silver-bearing solution.
The apparatus disclosed in U.S. Pat. No. 3,655,175 to Zeleny et al., Apr. 11, 1972, supports its media on a layer of inert mineral chips in an effort to overcome channelling, but this layer is still a discontinuous distributor resting on a perforated plate, so channelling is still not prevented.
The apparatus disclosed in U.S. Pat. No. 4,325,732 to Woog, Apr. 20, 1982, has its chamber filled with a fiberglass mat axially wound with media particles to keep the media discrete. This design purports to prevent channelling. However, the volume of the inert fiberglass reduces the amount of reactive media and therefore reduces substantially the overall capacity of the unit.
U.S. Pat. No. 4,662,613 to Woog, May 5, 1987, discloses a variant of his '732 apparatus which incorporates a combined radial and axial distributor and concentric spacers in an inlet distribution chamber to purportedly overcome unfavorable media distortions at the entry and exit of the unsupported media in the former apparatus. This design still suffers from a capacity penalty.